This invention relates generally to an apparatus for producing continuous glass filaments, and in particular, to an improved bushing including terminal ears that reduce the mechanical and thermal degradation experienced during the operation of the bushing and lengthen the operating life of the bushing and a method of manufacturing the improved bushing. The invention is particularly useful in the production of continuous glass filaments for use in a wide range of applications including textiles and reinforcements.
A strand of glass filaments is typically formed by attenuating molten glass through a plurality of orifices in a bottom plate of a bushing. The filaments are attenuated by applying tractive forces to the streams of glass, so as to attenuate the streams from the orifices in the bottom plate. The filaments are coated with a size or binder material that serves to provide a lubricating quality to the individual filaments to provide them with abrasion resistance or to impart a desired array of properties to the strand in its ultimate application. The size material is applied after the filaments are formed. The filaments are gathered in parallel relationship to form a strand.
Bushings condition the molten glass to a uniform temperature so the filaments are attenuated with uniform diameters. The temperature of the molten glass must be high enough to maintain the glass in a liquid state.
Bushings experience a corrosive operating environment imposed by the molten glass and elevated operating temperatures which accelerate the degradation of its electrical and mechanical components. One solution to the degradation is to fabricate the bushing assembly from precious metals such as platinum or platinum alloys. However, the operating environment affects these materials as well. Oxidation loss, volatilization, and migration of the precious metal into surrounding refractory materials as well as sagging or creep (high temperature deformation) of the bushing decrease bushing performance as well as shorten the useful life of the bushing assembly.
Conventional bushings typically include side plates, end plates, and a bottom plate defining a bushing body therebetween. The bottom plate may include more than 4,000 orifices or nozzles, preferably all at or close to a uniform temperature. The bottom plate may be referred to as a nozzle plate or tip plate as well.
Such bushings include terminals (referred to as xe2x80x9cterminal earsxe2x80x9d) coupled to each end plate. Both the bushing and the terminal ears are typically made of a precious metal, such as a platinum containing material like platinum or a platinum alloy. An example of a platinum alloy is a platinum-rhodium alloy. Electric clamps are connected to the terminals to deliver a heating current to the bushing to maintain the glass in its molten condition. The clamps are typically water-cooled. The current flows through the bushing side plates, end plates, and bottom plate which are conductive.
Bushing terminal ears have a wide range of shapes and are often wide and relatively thin. The terminal ears can extend substantially along the width of the bushing end plate and provide a relatively large area for engagement by an electrical clamp that supplies the current.
A common terminal ear design includes an upper portion coupled to the bushing and a lower portion to which electrical clamps are coupled.
In addition to tip section heat pattern, two design considerations for bushing terminal ears are the mechanical and thermal degradations of the terminal ears over time. During operation of the bushing, the upper terminal ears experience a combination of mechanical fatigue and thermal effects due to the high operating temperatures of the bushing.
The mechanical fatigue of the terminal ear is related to high levels of bending stress or vibrational fatigue during operation of the bushing. Since the ear is wide and thin, it tends to bend when the clamp is secured to it. A fissure in the terminal ear may begin and develop into a crack.
In addition to the mechanical fatigue, the terminal ear experiences thermal degradation. When some metals are heated to a high temperature, they volatize or evaporate. Platinum and rhodium are such metals. Since the bushing and the terminal ear operate at high temperatures, the platinum and rhodium present on the edges of the terminal ears gradually volatizes. The loss of alloy from the boundary of the ear promotes the formation of a fissure, which may expand into a crack. Mechanical working of the ear may increase the fissure and the terminal ear will eventually fail.
Several attempts have been made to solve the problem of cracking in the terminal ears. One design approach is to heal or repair cracks with molten wire, similar to soldering. This solution is a temporary fix and does not address the thermal degradation of the terminal ear.
Another design approach is to redesign the power delivery system to reduce vibration and stress on the ear. This can reduce but not eliminate the problem.
Another design approach involves the use of a gusset between a terminal ear and the end plate or the side plate of a bushing. U.S. Pat. No. 4,634,460 to Fowler (Fowler) discloses a drain bushing with a gusset in contact with each terminal ear to provide support for the ears. The ears and the gussets are integrally connected to the bottom plate of the bushing so that the heating current supplied by the clamps through the terminal ears is delivered to the bushing through the gusset and the terminal ears. The gussets effectively widen the contact between the terminal ears and the bushing. However, the gussets provide limited support to the terminal ear and the gusset. Also, the design approach of Fowler does not address the thermal degradation of the terminal ears.
Another design approach is to thicken the entire terminal ear to stiffen the ear. However, thicker terminal ears adversely effect the desired heating pattern of heat distribution because of undesirable low operating current levels.
A need exists for an economical way to improve the resistance of terminal ears to the mechanical and thermal degradations caused by the operating temperature of the bushing. Similarly, a need exists for an economical way to enhance the service life of a bushing.
The shortcomings of the prior art are overcome by the disclosed bushing including a terminal ear and the method of manufacturing the bushing. The bushing includes a bushing body (which can include side plates, end plates, a bottom plate) and terminal ears coupled to the bushing body. Clamps are attached to the terminal ears to supply electrical current to the bushing to maintain the glass inside in a liquid state and thermally condition it to be fiberized.
Each terminal ear includes an upper portion and a lower portion. The upper portion is coupled to a side plate of the bushing. A clamp is attached to the lower portion of the terminal ear. The upper and lower portions are oriented at an angle with respect to each other.
Support portions are provided along the side edges of the upper portions of the terminal ears. Support portions stiffen the terminal ear, thereby increasing its resistance to bending and fatigue stresses. Support portions also serve as heat sinks by providing additional mass and surface area at the edges of the terminal ears. Heat may be absorbed from the conducting portion by the support portions because of the additional mass. The increase in surface area facilitates the cooling of the edges of the terminal ear through radiation and convection.
Support portions do not contact the bushing end plate and therefore do not conduct current to the bushing body. Accordingly, the temperature at the edge of the support portions during operation of the bushing will be lower than the conducting portion of the terminal ears. Since the support portions operate at a lower temperature, the thermal degradation of the terminal ear is reduced.